Metal material with modified surface, preparation method and use of same

ABSTRACT

A metal material whose surface is modified by bonding of aromatic groups, optionally substituted by functional groups.

The invention relates to a metallic material, the surface of which ismodified, its preparation method and the use of the modified material.

It has already been shown that the electrochemical reduction ofdiazonium salts on a carbon electrode makes it possible to attach arylgroups covalently to the carbon surface.^([1]) This chemical reactioncan be carried out not only in an aprotic medium in acetonitrile, butalso in an aqueous acid medium.

The monolayers obtained with a great variety of diazonium salts havebeen characterized by various methods: cyclic voltammetry, photoelectronspectroscopy (XPS), vibration spectroscopy (PMIRRAS—PolarizationModulation InfraRed Reflexion Absorption Spectroscopy—and Ramanspectroscopy ^([2])), and Rutherford backscattering spectroscopy (RBS).Grafting occurs on the pendant bonds of the vitreous carbon, but also onthe base plane of the Highly Oriented Pyrolytic Graphite (HOPG) (i.e. onsp² carbons). On the basis of the well-known reduction of arylhalides,^([3]) this grafting reaction is attributed to the very reactivearyl radical produced during the electrochemical reduction of diazoniumsalts. The effect of these grafted organic layers on the rate ofelectron transfer has been measured.^([4]) These organic monolayers havebeen used for attaching redox enzymes^([Ib]) for limiting the adsorptionof proteins,[⁵] for differentiating dopamine from ascorbic acid inelectrochemical assays,^([6]) and for checking vitreous carbon fluxdetectors^([7]) The simple chemical reaction of the diazonium salts(without electrochemistry) has made it possible to attach aryl groups tocarbon blacks to produce materials for use in the plastics, rubber andtextiles industry.^([8]) It is also possible to obtain compact layers ofaryl groups on silicon by electrochemical reduction of diazonium saltson hydrogenated silicon Si (111).^([9])

Up to now, grafting of aromatic groups on the surface of metallicmaterial has not been carried out.

The invention relates in particular to the grafting of aromatic groupson the surface of metallic material.

The invention relates in particular to grafting that is stable overtime, of aromatic groups on the surface of a metallic material.

In one of its generalities, the invention relates to a metallic materialwhose surface is modified by bonding aromatic groups to the saidsurface, optionally substituted by functional groups.

It was found, unexpectedly, that it was possible to form stablemetal-carbon bonds, the said carbon being inserted in an aromatic group.

By “aromatic group” is meant a radical derived from a cyclic compoundhaving one or more independent or condensed benzene rings, and/or one ormore complex rings derived from benzene. This radical can of course alsocontain heterocyclic rings and/or various substituents as well ashydrocarbon chains optionally containing heteroatoms such as N, O and S.

The expression “metallic material whose surface is modified by bondingof aromatic groups to the said surface” means that a layer that can havea minimum thickness of about 10 Å, i.e. is a monolayer, is grafted onthe surface of the metallic material.

In general, the layers grafted on the surface of the metallic materialaccording to the invention have a thickness varying from about 10 Å toabout 1 mm.

The expression “aromatic groups optionally substituted by functionalgroups” means that the grafted aromatic groups can undergo subsequentchemical transformations, as will become evident in the course of thedescription that follows.

In the metallic material according to the invention, the nature of thebond between the surface and the aromatic groups modifying it is acarbon-metal bond of the covalent type, and in particular is such thatit withstands ultrasonic cleaning.

The expression “carbon-metal of the covalent type” means a strongnon-ionic bond obtained by overlapping of the orbitals of the carbon andof the metal.

The conditions of resistance of the carbon-metal bond to ultrasoniccleaning are described by way of illustration, in the examples givenlater.

According to an advantageous embodiment, in the metallic materialaccording to the invention, the aromatic group is a C₆–C₁₄ aromaticresidue, optionally substituted by one or more functional substituentsor a heteroaromatic residue of 4 to 14 atoms, optionally substituted byone or more functional substituents, having one or more heteroatomsselected from oxygen, nitrogen, sulphur or phosphorus.

According to another embodiment, in the metallic material according tothe invention, the aromatic group contains one or more substituentsselected from the group comprising:

-   -   linear or branched aliphatic radicals of 1 to 20 carbon atoms,        optionally containing one or more double or triple bonds,        optionally substituted by the following radicals: carboxyl, NO₂,        disubstituted protected amino, monosubstituted protected amino,        cyano, diazonium, alkoxy of 1 to 20 carbon atoms, alkoxycarbonyl        of 1 to 20 carbon atoms, alkylcarbonyloxy of 1 to 20 carbon        atoms, vinyl optionally fluorinated or allyl, halogen atoms,    -   aryl radicals optionally substituted by the radicals carboxyl,        NO₂, disubstituted protected amino, monosubstituted protected        amino, cyano, diazonium, alkoxy of 1 to 20 carbon atoms,        alkoxycarbonyl of 1 to 20 carbon atoms, alkylcarbonyloxy of 1 to        20 carbon atoms, vinyl optionally fluorinated or allyl, halogen        atoms,    -   the following radicals: carboxyl, NO₂, disubstituted protected        amino, monosubstituted protected amino, cyano, diazonium, alkoxy        of 1 to 20 carbon atoms, alkoxycarbonyl of 1 to 20 carbon atoms,        alkylcarbonyloxy of 1 to 20 carbon atoms, vinyl optionally        fluorinated, halogen atoms.

According to another embodiment, the metallic material according to theinvention is such that the aromatic group contains one or moresubstituents capable of reacting directly with a substrate, or one ormore precursor substituents which, after transformation, are capable ofreacting with a substrate, the said substrate being selected from thegroup comprising organic resins, biological molecules, chemicalmolecules or complexing agents.

According to another embodiment, the metallic material according to theinvention is such that the aromatic group contains one or moresubstituents capable of reacting directly with an organic resin andselected from the group comprising allyl or vinyl groups,—(CH₂)_(n)—COOH, —(CH₂)_(n)—CH₂—OH, (CH₂)_(n)—NH₂, n being an integerbetween 0 and 10, or one or more precursor substituents capable ofreacting after transformation with an organic resin and selected fromthe group comprising NO₂, N₂ ⁺, (CH₂)_(n)—CN, (CH₂)_(n)—CHO,(CH₂)_(n)—COOPr, Pr being a protective group, (CH₂)_(n)—NHP′r,(CH₂)_(n)—N(P′r)₂, (CH₂)_(n)—N═P″r, P′r and P″r being protective groups,and n being an integer between 1 and 10.

According to another embodiment, the metallic material according to theinvention is such that the aromatic group contains one or moresubstituents capable of reacting directly with a biological molecule andselected from the group —(CH₂)_(n)—COOH, (CH₂)_(n)—NH₂, n being aninteger between 0 and 10, or one or more precursor substituents capableof reacting after transformation with a biological molecule and selectedfrom the group comprising NO₂, N₂ ⁺, (CH₂)_(n)—CN, (CH₂)_(n)—CHO,(CH₂)_(n)—COOPr, Pr being a protective group, and n being an integerbetween 0 and 10.

According to another embodiment, in the metallic material according tothe invention, the aromatic group contains one or more substituentscapable of reacting directly with functional organic molecules selectedfrom the group comprising NO₂, (CH₂)_(n)—CONH₂, (CH₂)_(n)—CN,(CH₂)_(n)—CHO, (CH₂)_(n)—COOH, (CH₂)_(n)—CH₂OH, (CH₂)_(n)—NH₂, n beingan integer between 0 and 10, SO₂H, SO₃H, SO₂R, SO₃R, R being analiphatic or aromatic carbon chain of 1 to 20 carbon atoms, or one ormore precursor substituents capable of reacting after transformationwith functional organic molecules selected from the group comprisingNO₂, (CH₂)_(n)—CONH₂, (CH₂)_(n)—COOPr, Pr being a protective group,(CH₂)_(n)—NHP′r, (CH₂)_(n)—N (P′r)₂, (CH₂)_(n)—N═P″r, P′r and P″r beingprotective groups, (CH₂)_(n)—CN, (CH₂)_(n)—CHO, (CH₂)_(n)—COOH,(CH₂)_(n)—CH₂OH, n being an integer between 0 and 10, SO₂Pr, SO₃Pr, Prbeing a protective group selected from the meanings of R.

According to another embodiment of the invention, the aromatic group isselected from:

Hereinafter, the diazonium salts will be identified by their number, asindicated above.

The following compounds are used advantageously:

since their substituents can easily be characterized by cyclicvoltammetry (NO₂, quinone), by XPS or RBS (NO₂, I, COOH) and PMIRRAS(NO₂). The other diazoniums were selected for their possible hydrophobicproperties that can lead to a decrease in the rates of corrosion.

According to an advantageous embodiment of the invention, the metal isselected from pure metals or alloys, and especially iron, nickel,platinum, gold, copper, zinc, cobalt, titanium, chromium, silver,stainless steels, titanium alloys, chromium-cobalt alloys, molybdenum,manganese, vanadium.

For preparation of the metallic material according to the invention, itis possible to employ a method consisting of fixing an aromatic group onthe surface of the said material, by electrochemical reduction of adiazonium salt containing this aromatic group, by placing the metallicmaterial in contact with a solution of the diazonium salt in a solventand negatively polarizing the metallic material relative to an anodethat is also in contact with the solution of the diazonium salt, itbeing optionally possible for the anode and the cathode to be separatedfrom one another, for example by a diaphragm or a membrane.

When aryl groups are grafted on carbon, carbon-carbon bonds areobtained, which are the usual bonds of organic chemistry. On vitreouscarbon, bonding probably occurs on account of the pendant bonds, and onHighly Oriented Pyrolytic Graphite by transformation of an sp² carbon toan sp³ carbon. As for iron-carbon or more generally metal-carbon bonds,they are far less usual and moreover nothing would suggest that thereduction of the diazonium salt on the metal of the electrode might leadto strong fixation of aromatic groups on the surface. Moreover, it isunusual to reduce organic substrates on less noble metals like iron orzinc or even copper and titanium; as for the noble metals, they arecommonly employed as electrodes in molecular electrochemistry, preciselybecause they do not react with the substrates, the reactionintermediates or the end products. The method of the invention could nottherefore be envisaged on the basis of present knowledge.

In this method, if we assume that the diazonium salt corresponds to theformula ArN⁺ ₂X⁻, where Ar represents the aromatic group of thediazonium salt and X⁻ represents an anion, the electrochemical reductionof the diazonium cation ArN⁺ ₂ corresponds to the following reactionscheme:ArN⁺ ₂+1e ⁻→ArN₂→Ar+N₂

Thus, reduction of the diazonium cation releases the aromatic group fromthe diazonium salt in the form of a free radical that can attack thesurface of the metallic material and become fixed on it.

The diazonium salts were chosen because they are more easily reduciblethan the radical to which they give rise. According to the invention, itis therefore necessary for the reduction potential of the diazonium saltused to be less negative than the reduction potential of the freeradical Ar⁻ corresponding to the aromatic group of the said diazoniumsalt.

In fact, if the reduction potential of the diazonium salt was morenegative than the reduction potential of the radical Ar³¹ , the freeradical Ar⁻ would be reduced to Ar⁻ during electrochemical reduction ofthe diazonium salt and Ar⁻ would be protonated either by residual water,or by the solvent itself to form ArH, without fixing itself to themetallic material.

This latter condition concerning the reduction potential of thediazonium salt is fulfilled for nearly all the diazonium salts.

According to one embodiment of the invention, the anode and the cathodeare not separated.

In this case the anode and the cathode are immersed in the solutioncontaining the diazonium salt. Two electrodes are sufficient for workingin intentiostatic mode. A reference electrode must be added, immersed inthe same solution, if working in potentiostatic mode.

According to yet another embodiment, the anode and cathode areseparated, for example by a diaphragm or a membrane.

In this case, just two electrodes are required when working inintentiostatic mode. If working in potentiostatic mode it is necessaryto add a reference electrode in the cathode compartment.

The invention also relates to a method in which electrochemicalreduction takes place in the presence of an electrolyte, the anode andcathode compartments optionally being separated, with the anodecompartment containing the solvent and the electrolyte, and the cathodecompartment containing the solvent, the electrolyte and the diazoniumsalt.

Electrochemical reduction can take place in the presence of anelectrolyte.

According to this configuration, the anode and the cathode need not tobe separated.

Both are immersed in the solution. Two electrodes are sufficient inintentiostatic mode. A reference electrode must be added inpotentiostatic mode.

According to another embodiment of the invention, the anode and thecathode can be separated, by a membrane or a diaphragm. Two electrodesare sufficient in intentiostatic mode. In potentiostatic mode, on theother hand, it is necessary to add a reference electrode in the cathodecompartment.

According to an advantageous embodiment, in the method according to theinvention, the diazonium salt corresponds to the formula ArN₂ ⁺ X⁻, inwhich Ar represents the aromatic group and X represents an anion and inthat the diazonium salt has a reduction potential that is less negativethan the reduction potential of the free radical Ar° corresponding tothe aromatic group of the diazonium salt, the X⁻ anion of the diazoniumsalt being selected advantageously from the halogens, the sulphates, thephosphates, the perchlorates, the tetrafluoroborates, the carboxylates,and the hexafluorophosphates.

According to another embodiment, in the method according to theinvention, reduction is carried out by repetitive cyclic voltammetry ina potential range in which the diazonium salts are reduced either byelectrolysis at a potential that is more negative than the reductionpotential of the diazonium salt, or at constant current (intentiostaticmode).

When reduction of the diazonium salt is carried out by repetitive cyclicvoltammetry in a potential range in which the diazonium salts arereduced

Successive scans are carried out between a potential that is lessnegative than the reduction wave of the diazonium salt and a potentialthat is more negative than the reduction wave.

When reduction of the diazonium salt is carried out at a potential thatis more negative than the reduction potential of the diazonium salt

The potential of the cathode is then positioned at the level of thereduction wave of the diazonium salt or at a potential that is morenegative than the potential of the reduction wave of the diazonium salt.When reduction is carried out in intentiostatic mode, the electrolysiscurrent is set to a value such that only the diazonium salt is reduced.

According to another embodiment, in the method according to theinvention the concentration of diazonium salts is between 10⁻³ and 10⁻¹mol/l.

According to another embodiment, in the method according to theinvention for producing, by electrochemical means, a metallic materialwhose surface is modified by aromatic aminated groups, the aromaticdiazonium salt is substituted by a nitro radical and electrochemicalreduction is maintained until there is reduction of the nitro radical toamino radical in an aqueous acid medium.

According to another embodiment, in the method according to theinvention, electrochemical reduction of the diazonium salt takes placein an aprotic solvent, in particular selected from the group comprisingacetonitrile, dimethylformamide, dimethylsulphoxide and benzonitrile.

According to another embodiment, in the method according to theinvention, the solution of the diazonium salt contains a supportingelectrolyte consisting of a quaternary ammonium salt or a lithium salt,especially a tetraalkylammonium tetrafluoborate.

In the method according to the invention, electrochemical reduction ofthe organic diazonium salt can take place in a protic solvent in an acidmedium.

According to another embodiment of the invention, the protic solvent isselected, from the group comprising water, methanol, ethanol or mixturesthereof, or in that the protic solvent is mixed with an aprotic solvent,it being understood that the resulting mixture has the characteristicsof a protic solvent.

According to another embodiment, in the method according to theinvention, the acid is selected from sulphuric, hydrochloric, nitric,nitrous, phosphoric or tetrafluoroboric acids.

In the method according to the invention, according to anotheradvantageous embodiment, the pH of the solution is less than 2.

The invention also relates to a metallic material as obtained byimplementation of the method described above.

The metallic material according to the invention can be characterized bythe fact that the bond between its surface and the aromatic groupsmodifying it is such that when an anode sweep is carried out startingfrom the corrosion potential of the metal forming the surface of themetallic material, there is at least one potential responsible for thedissolution of a detectable quantity of the aforesaid metal but whichdoes not disrupt the aforesaid bond.

The metallic material according to the invention can also becharacterized by the fact that the nature of the bond between itssurface and the aromatic groups modifying it is such that when an anodesweep is applied ranging from the corrosion potential to a more anodicpotential of about 75 mV, to the metallic material whose surface ismodified, rupture of the aforesaid bond does not occur, but there isdissolution of a detectable quantity of the metal forming the surface ofthe metallic material.

The diazonium salt is either prepared separately and prior to itsaddition, in a reactor, with a view to modification of the surface ofthe metallic material, or it is prepared in situ, by bringing together,in a reactor, the components necessary for its formation, according tothe standard methods of the prior art.

The invention also relates to any use of the metallic materials definedabove and in particular the following uses:

1-Protection of metals against corrosion either by layers obtaineddirectly or by polymer layers obtained from polymerizable substituents,for example:

2-Lubrication by placing, on the metallic surface, molecules thatimprove the friction properties (e.g. perfluorinated molecules) ormolecules possessing great affinity for the lubricant (e.g. long alkanechains with affinity for mineral oils and greases)

3-Adhesion of two metallic surfaces, each being modified by the abovemethod, it being possible for the R and R′ groups to react chemicallywith one another.

4-Improvement of biomaterials by covering the metallic surface(titanium, titanium alloys, stainless steel, chromium-cobalt alloys) forexample with:

-   -   chains of the polyethyleneglycol type replacing the aromatic        ring that would prevent the adhesion of proteins;    -   proteins such as bone morphogenic protein, which is able to        stimulate bone growth;    -   antibiotics replacing the aromatic group to form bactericidal        surfaces.

FIGURE CAPTIONS

In what follows, the abbreviations have the following meanings:

SCE: saturated calomel electrode

d: electrode diameter

v: sweep rate

ACN: acetonitrile

c: concentration

Z_(Im): imaginary part of the impedance

Z_(Re): real part of the impedance

FIGS. 1 a, 1 b, 1 c. Cyclic voltammogram in ACN+0.1 M NBu₄BF₄ a) ofnitrobenzene (concentration=2 mM) on a vitreous carbon electrode, b) onan iron electrode and c) of an iron electrode modified by 4-nitrophenylgroups, v=0.2 V/s. Reference SCE.

FIGS. 2 a, 2 b. Cyclic voltammogram of a) an iron electrode modified byanthraquinone groups and transferred to a solution ACN+0.1 M NBu₄BF₄ andb) an iron electrode in a solution ACN+0.1 M NBu₄BF₄+3 mM anthraquinone.v=0.2 V/s. Reference SCE.

FIGS. 3 a, 3 b. Cyclic voltammograms in a solution ACN+0.1 M NBu₄BF₄ onan iron electrode of d=3 mm a) that has been modified by anthraquinonegroups in 0.1N H₂SO₄ and b) in a 2 mM solution of anthraquinone (theelectrode was immersed beforehand for five minutes in 0.1N H₂SO₄ thenrinsed).

FIGS. 4 a, 4 b, 4 c. PMIRRAS spectra of plates of mild steel a)polished, b) modified in ACN by 4-nitrophenyl groups and c) modified in0.1N H₂SO₄ by 4-nitrophenyl groups.

FIG. 5. RBS spectrum of a plate of mild steel modified (in ACN+0.1 MNBu₄BF₄) by 4-iodophenyl groups.

FIGS. 6 a, 6 b, 6 c, 6 d. XPS spectra of: a) a clean plate of mildsteel, b) a mild steel plate grafted with 4-nitrophenyl groups, c) with4-carboxyphenyl groups and d) with 4-iodophenyl groups.

FIG. 7. Diagram of impedance in 0.1N H₂SO₄ of a) an iron electrode andb) an iron electrode modified by 4-hexadecyloxyphenyl groups.

FIGS. 8 a, 8 b, 8 c. Voltammograms in ACN+0.1M NBu₄BF₄ of a) a carbonelectrode in an anthracene solution (concentration=2 mM), b) a zincelectrode in an anthracene solution (concentration=2 mM) and c) a zincelectrode, grafted (in ACN+0.1M NBu₄BF₄+2 mM 10) with anthracenylgroups. v=0.2 V/s.

FIGS. 9 a, 9 b. Voltammogram in ACN+0.1M NBu₄BF₄ of a) a carbonelectrode in the presence of nitrobenzene, b) a zinc electrode, grafted(in ACN+0.1M NBu₄BF₄+2 mM 1) with 4-nitrophenyl groups.

FIGS. 10 a, 10 b. Cyclic voltammetry on a copper electrode in a solutionACN+0.1M NBu₄BF₄ a) in the presence of anthraquinone (concentration=2mM) and b) after grafting of the electrode (in ACN+0.1M NBu4BF₄+2 mM 11)with anthraquinone groups. Reference SCE, v=0.2 V/s

FIGS. 11 a, 11 b. Cyclic voltammetry on a copper electrode in a solutionACN+0.1M NBu₄BF₄ a) in the presence of nitrobenzene (concentration=2 mM)and b) after grafting of the electrode (in ACN+0.1M NBu₄BF₄+2 mM 1) with4-nitrophenyl groups. Reference SCE, v=0.2 V/s

FIGS. 12 a, 12 b, 12 c. Cyclic voltammetry on a nickel electrode in asolution ACN+0.1M NBu₄BF₄ a) in the presence of diazonium salt 1(concentration=2 mM), b) in the presence of nitrobenzene(concentration=2 mM) and c) after grafting of the electrode (in ACN+0.1MNBu₄BF₄+2 mM 1) with 4-nitrophenyl groups. Reference SCE, v=0.2 V/s

FIGS. 13 a, 13 b. Cyclic voltammetry of a cobalt electrode a) in asolution ACN+0.1M NBu₄BF₄ b) after grafting of the electrode (inACN+0.1M NBu₄BF₄+2 mM 1) with 4-nitrophenyl groups. Reference SCE, v=0.2V/s.

FIGS. 14 a, 14 b, 14 c. Cyclic voltammetry on a gold electrode in asolution ACN+0.1M NBu₄BF₄, a) in the presence of 4-nitrobenzenediazoniumtetrafluoroborate 1, b) in the presence of nitrobenzene (concentration=2mM) and c) after grafting of the electrode (in ACN+0.1M NBu₄BF₄+2 mM 1)with 4-nitrophenyl groups. Reference SCE, v=0.2 V/s

FIGS. 15 a, 15 b, 15 c. Cyclic voltammetry on a platinum electrode in asolution ACN+0.1M NBu₄BF₄, a) in the presence of 4-nitrobenzenediazoniumtetrafluoroborate 1, b) in the presence of nitrobenzene (concentration=2mM) and c) after grafting of the electrode (in ACN+0.1M NBu₄BF₄+2 mM 1)with 4-nitrophenyl groups. Reference SCE, v=0.2 V/s.

FIGS. 16 a, 16 b. Cyclic voltammetry on a titanium electrode in asolution ACN+0.1M NBu₄BF₄ a) in the presence of 4-nitrobenzenediazoniumtetrafluoroborate 1 (concentration=2 mM) and b) after grafting of theelectrode (in ACN+0.1M NBu₄BF₄+2 mM 1) with 4-nitrophenyl groups.Reference SCE, v=0.2 V/s

FIGS. 17 a, 17 b, 17 c. Cyclic voltammetry on a stainless steelelectrode in a solution ACN+0.1M NBu₄BF₄, a) in the presence of4-nitrobenzenediazonium tetrafluoroborate 1, b) after grafting of theelectrode (in ACN+0.1M NBU₄BF₄) with 4-nitrophenyl groups, and c) in thepresence of nitrobenzene (concentration=2 mM). Reference SCE, v=0.2 V/s

FIG. 18. Cyclic voltammetry in a solution ACN+0.1M NBu₄BF₄ of a) an ironelectrode in the presence of anthracene (c=2.5 mM) and b) an ironelectrode grafted with anthracenyl groups. Reference SCE, v=0.2 V/s.

FIG. 19. Cyclic voltammogram on an iron electrode (diameter 1 mm) in asolution ACN+0.1M NBu₄BF₄ in the presence of nitrobenzene(concentration=1 mM). Reference SCE. v=0.1 V/s. Scale of the abscissaaxis: 100 mV/cm. Scale of the ordinate axis: 5 μA/cm (Comparativeexample No. 1)

FIG. 20. Cyclic voltammogram on the preceding iron electrode; this testis conducted after obtaining the voltammogram of FIG. 19 and with thesame electrode as that used for FIG. 19 (after thorough rinsing in anultrasonic tank) in a solution of ACN+0.1M NBu₄BF₄. Reference SCE. v=0.1V/s. Scale of the abscissa axis: 100 mV/cm. Scale of the ordinate axis:0.8 μA/cm (Comparative example No. 1)

FIGS. 21A, 21B. Cyclic voltammogram on an iron electrode (diameter 1 mm)in a solution of ACN+0.1M NBu₄BF₄ in the presence of p-nitrophenol(concentration=1 mM). Reference SCE. v=0.3 V/s. Curve (A) corresponds tothe electrode blank and curve (B) to the voltammogram in the presence ofp-nitrophenol. Scale of the abscissa axis: 100 mV/cm. Scale of theordinate axis: 0.8 μA/cm (Comparative example No. 2)

FIG. 22. Cyclic voltammogram on the preceding electrode (see FIGS. 20Aand 20B) in a solution of ACN+0.1M NBu₄BF₄; this test is conducted afterobtaining the voltammogram of FIG. 21B and thorough rinsing of thiselectrode in an ultrasonic tank. Reference SCE. (a) v=0.1 V/s and (b)v=0.3 V/s. Scale of the abscissa axis: 100 mV/cm. Scale of the ordinateaxis: 0.8 μA/cm (Comparative example No. 2)

EXAMPLES

Experimental Section

Chemical products, Electrodes. The ACN is from Merck (Uvasol), H₂SO₄from Prolabo (Tritrinorm) and NBu₄BF₄ from Fluka (puriss.). Thediazonium salts: 1 and 11 are of commercial origin (Aldrich), synthesisof 3 has been described^([1c]); 2, 3–6, 9, 10 were obtained startingfrom commercial amines by standard methods^([27]). The diazonium saltsare stored in a refrigerator.

4-iodobenzene diazonium tetrafluoroborate 2 ^([28]): M.p. 126° C. (inthe literature 123–124° C.); ¹H NMR (200 MHz, DMSO): δ=8.4 (q, 4H,aromatics)

4-methylbenzene diazonium tetrafluoroborate 4^([29]): M.p. 110° C.,decomp. (in the literature 110° C., decomp.); ¹H NMR (200 MHz, DMSO):δ=2.5 (q, 3H, CH₃), 7.8–8.5 (two d, 4H, aromatics).

4-n-butylbenzene diazonium tetrafluoroborate 5^([28]): In an Erlenmeyerplaced in an ice bath, dissolve 4-n-butyl aniline (149 mg, 1 mmol) inHBF₄ (34%, d=1.23, 3 mmol, 30 mL) and stir for 15 minutes, then addNaNO₂ (1.5 mmol, 103 mg). Stir the mixture for 20 minutes. Then addCH₂Cl₂ (30 mL) and separate the organic and aqueous layers. Dry theorganic phase over MgSO₄ and evaporate to give a pale yellow solid. Asindicated in the literature, it is not possible to measure a meltingpoint^([28]); ¹H NMR (200 MHz, DMSO): δ=0.9 (t, ³J=6 Hz, 3H, CH₃),0.8–2.8 (m, 5H, CH₂), 2.8 (t, ³J=8 Hz, 2H, benzyl CH₂), 7.6–8.5 (q, ³J=8Hz, 4H, aromatics).

4-n-dodecylbenzene diazonium tetrafluoroborate 6^([30]) M.p. 70° C.,decomp.; ¹H NMR (200 MHz, DMSO): δ=0.85 (t, ³J=6 Hz, 3H, CH₃), 1.1–2.8(m, 20H, CH₂), 2.5 (t, ³J=8 Hz, 2H, benzyl CH₂), 7.8–8.5 (q, ³J=8 Hz,4H, aromatics).

Diazonium salt 7 was prepared from 4-dodecyloxyaniline^([31]): whitesolid, ¹H NMR (200 MHz, DMSO): δ=0.8–1.8 (m, 25H, aliphatic protons),4.3 (t, 2H, O—CH₂), 7.5–8.5 (q, ³J=8 Hz, 4H, aromatics).

Diazonium salt 8 was prepared in the same way, except that the diazoniumsalt contained a little more of white solid amine: ¹H NMR (200 MHz,DMSO): δ=0.8–1.8 (m, 33H, aliphatic protons), 4.2 (t, 2H, O—CH₂),7.2–8.5 (q, ³J=8 Hz, 4H, aromatics)

The electrodes were prepared from metal wire, for example from iron wirewith a diameter of 1 mm (Johnson Matthey 99.99%) sealed in epoxy resinor from buttons of soft iron with diameter of 3 mm held in a Teflonclamp. The plates of mild steel (containing 95.68% Fe, 0.31% C, 2.03%Mn, 0.05% P, 0.13% S, 0.56% N, 0.10% Si, 0.07% Cu, 0.18% Ni, 0.30% Cr,0.01% Sn, 0.58% Al) were donated by the Sollac company.

Electrochemical equipment: The electrochemical curves were obtainedusing a potentiostat constructed in the laboratory or using a VersastatII system from EGG. The impedance measurements were obtained with an EGG263A potentiostat and a 5210 EGG lock-in amplifier. The frequenciesinvestigated varied from 0.001 Hz to 20 kHz and the corrosion currentswere obtained from Tafel straight lines with EGG corrosion software.

Surface characterization: The equipment for XPS, PMIRRAS and RBS and theconditions of use have been described previously [1 c].

Example I

Grafting of Iron

Electrochemical tests of grafting. The grafting reaction wasinvestigated both in an aprotic medium [acetonitrile (ACN)+0.1M NBu₄BF₄]and in an aqueous acidic medium (the diazonium salts are not stable inan aqueous medium above pH 2). In ACN+0.1M NBu₄BF₄ an iron electrode hasa range of electro-activity that is between −0.1 and −2.3 V/SCE; indilute sulphuric acid (0.1 N) this range is reduced to nothing. Theopen-circuit voltage (corrosion voltage)^([10]) is located at −0.58V/SCE and corresponds to a mixed voltage since the reduction andoxidation reactions are different. The anode or cathode partial currentthat flows at this voltage is called the corrosion current (I_(corr)).As soon as the voltage is displaced to positive voltages, the oxidationcurrent of the iron predominates, whereas the reduction of protons or ofoxygen predominates when the voltage is shifted towards negative values^([11]). It should be recalled that even at the corrosion voltage, wherethe current is zero, the phenomena of oxidation of the iron andreduction of the protons and/or of the oxygen take place simultaneously.At the corrosion voltage, the two currents are equal. The region ofelectro-activity is therefore completely different from that of a carbonelectrode and it is not always possible, as we have describedpreviously,^([1]) to observe the reduction wave of the diazonium saltslocated near 0 V/SCE.

In ACN, the only general way of grafting aryl groups on the surface ofiron is to set the voltage to a value more negative than −0.5 V/SCE in asolution containing the diazonium salt and to a value close to thecorrosion voltage in 0.1N H₂SO₄ (we shall see later that it is possibleto determine an optimum voltage of −0.75 V/SCE in this solvent). Indoing this, it is assumed that the diazonium salts are reduced atsimilar voltages on carbon and on iron. The reduction potential of thediazonium salts and the oxidation potential of iron in an aqueous mediummakes it thermodynamically possible for diazonium salts to be reduced byiron. We shall see later that XPS measurements show that thisspontaneous reaction is negligible for grafting on iron, but spontaneousgrafting on stainless steel has been observed.

The reduction of 4-nitrobenzene diazonium tetrafluoroborate 1, which, oncarbon, leads to grafting of 4-nitrophenyl groups was investigated.These groups are readily characterized by their reversible reductionwave in an aprotic medium which is located at the same potential as thatof nitrobenzene. First of all we recorded the cyclic voltammogram ofnitrobenzene in ACN+0.1M NBu₄BF₄ on an iron electrode; two waves areobserved, at −1.22 and −1.75 V/SCE (not shown), the height of the secondbeing almost double that of the first. The first wave is reversiblesince an anode wave is observed at −1.00 V/SCE during the return sweep.The standard potential of nitrobenzene can therefore be measured: °E=−1.11 V/SCE. This voltammogram is similar to that observed on carbonelectrodes (FIGS. 1 a, 1 b).

Next, the voltage of an electrode is maintained at −0.8 V/SCE for 5minutes in a solution of ACN+0.1M NBu₄BF₄ containing diazonium salt 1(c=2 mM). The electrode is then rinsed carefully in an ultrasonic tankfor five minutes then transferred to a fresh solution containing onlythe solvent and the supporting electrolytes. In these conditions, abroad, reversible wave is observed (E_(pc)=−1.25 V/SCE and E_(pa)=−1.12V/SCE) at a potential very close to that of nitrobenzene itself (FIG. 1c). This experiment clearly shows that the 4-nitrophenyl groups aretransferred with the electrode and that they are firmly fixed since theywithstand cleaning in an ultrasonic tank. FIGS. 2 a and 2 b show theresults obtained with diazonium salt 11 which is a commercial diazoniumsalt of anthraquinone. The iron electrode, which was modified andcarefully rinsed as above, exhibits a reversible system with a cathodepeak at E_(pc)=−0.93 V/SCE and an anode peak at E_(pa)=−0.88 V/SCE whichcorresponds to a standard potential E°=−0.90 V/SCE. By comparison,anthraquinone exhibits on an iron electrode, a slow electrochemicalsystem (E_(pc)=−1.08 V/SCE and E_(pa)=−0.75 V/SCE) at ° E=−0.91 V/SCE,i.e. at the same potential as the modified electrode. This experimentconfirms the observations made with diazonium salt 1. Moreover, thesymmetrical shape of the voltammogram indicates a redox system withoutdiffusion from and to the electrode as was to be expected for groupsgrafted to the surface of the electrode. Attention should also be drawnto the great stability of the system during the successive sweeps.Similar results were obtained with anthracenyl groups bound to thesurface by reduction of diazonium salt 10 (FIG. 18).

Electrochemical modification of a vitreous carbon electrode with4-nitrophenyl groups was carried out previously by reduction ofdiazonium salt 1 in dilute H₂SO₄ and the signal of the 4-nitrophenylgroups was observed in a solution of ACN+0.1M NBu4BF₄. Such a resultcannot be observed with an iron electrode that has been modified byreduction of diazonium salt 1. This is because during electrolysis ofdiazonium salt 1 in an acid medium at the grafting voltage, E=−0.75V/SCE, the nitro group is reduced to amine (the voltammogram ofnitrobenzene on a vitreous carbon electrode under the same conditions asabove corresponds to the transfer of 6e⁻+6H⁺), a function that cannot bereduced electrochemically and so cannot be observed by cyclicvoltammetry. However, an iron electrode modified by reduction ofdiazonium salt 11 at E=−0.5 V/SCE in 0.1N H₂SO₄, carefully rinsed andtransferred to a solution of ACN+0.1M NBu₄BF₄, displays a weaklyreversible voltammogram (E_(pc)≈−0.9 V/SCE and E_(pa)≈−0.6 V/SCE) at °E≈−0.7 V/SCE. This voltammogram (FIG. 3) is similar to that ofanthraquinone in ACN+0.1M NBu₄BF₄ on an iron electrode previously heldin a solution of 0.1N H₂SO₄ for five minutes. These experiments in anacid medium show that it is possible to graft aryl groups on the surfaceof iron but the voltammograms obtained are different from those obtainedby grafting in ACN. This is evident from a loss of reversibility andslower electron transfer. The loss of reversibility may be due to acidicspecies transferred with the electrode despite vigorous rinsing.

Modification of the surface of iron by reduction of diazonium salts istherefore possible in ACN and aqueous acidic solutions, but it is moreeasily observed in ACN. These results were then confirmed by othermethods.

Vibration spectra of the organic layer: As in the case of organicmonolayers on carbon, it is difficult to obtain infrared spectra ofmonolayers by reflection. However, this can be achieved by PMIRRAS(Polarization Modulation InfraRed Absorption Spectroscopy ^([12])). Thespectra were recorded on plates of mild steel (an industrial materialthat needs to be protected against corrosion). Before grafting, theplates were carefully polished and rinsed in the absence of oxygen in anultrasonic tank. An ungrafted plate exhibits absorptions at 1050–1100cm⁻¹ that are characteristic of oxides as well as absorptions at 1400and 1600–1700 cm⁻¹ that correspond to carbonates and tohydrogen-carbonates (which probably arise from reaction with atmosphericcarbon dioxide after grafting) (FIG. 4 a). After grafting with diazoniumsalt 1 in a solution of ACN+0.1M NBu₄BF₄ the symmetrical andantisymmetrical vibrations of the NO₂ group ^([13]) are clearly visibleat 1350 and 1522 cm⁻¹ as well as the absorptions of the aromatic groupat 1263 and 1600 cm⁻¹ (FIG. 4 b). The peaks of the oxides can be seen,but those of the carbonates have disappeared. If grafting is carried outin 0.1N sulphuric acid, a strong band is observed at 1180 cm⁻¹ which maybe due to a thicker layer of oxide or to the presence of sulphates,however, the signal of the NO₂ group can still be observed (FIG. 4 c).

Rutherford Backscattering Spectra (RBS): This method, which measures thenumber of He⁺ ions backscattered at a given energy, permitscharacterization of the presence of a given type of atoms on thesurface, as well as their number. It is, however, necessary for theatomic number of the atom being identified to be higher than that ofiron. To fulfil this condition, we grafted carefully polished mild steelplates by reduction of 4-iodobenzene diazonium tetrafluoroborate in ACN.The spectrum of this plate is shown in FIG. 5. Table 1 shows the resultof various tests as a function of the solvent and of the voltage. Theonly peak observed apart from that of iron corresponds to the iodineatoms, providing good confirmation of grafting of the iodated molecule.Furthermore, analysis shows that grafting is uniform on the wholesurface of the specimen and remains stable under the ion beam. In dilutesulphuric acid the optimum value of the voltage is −0.75 V/SCEcorresponding to a maximum surface concentration; at more positivepotentials, grafting competes with oxidation of the surface and withreduction of protons at more negative potentials. The surfaceconcentrations will be discussed later.

X-ray Photoelectron Spectroscopy (XPS): It should be noted that RBS andXPS are complementary methods of surface analysis since the depth ofsampling in the first is about two orders of magnitude less than in thesecond. FIGS. 6 a, 6 b, 6 c and 6 d show the complete spectrum of aplate of mild steel (FIG. 6 a) and of plates grafted with 4-nitrophenylgroups (FIG. 6 b), 4-carboxyphenyl groups (FIG. 6 c) and 4-iodophenylgroups (FIG. 6 d). All these general spectra exhibit the peaks C_([1s]),O_([1s]), F_([2p]) at energies of 285, 530 and 710 eV. FIG. 6 b clearlyshows the grafting of diazonium salt 1 after electrochemical treatment.This grafting is reflected in a definite increase in relative intensityof the C[1s] peak and a small peak at about 400 eV. The enlargement ofthis region reveals two peaks centred at 400 and 406 eV, the lattercorresponding to the nitro group, whereas the first may be due either tocontamination or to products of reduction of the nitro group. It shouldbe pointed out that the O[1s] peak has been adjusted with a component at533 eV, a characteristic energy of the oxygen of a nitro group^([)14],at the same time as with a component with lower bond energy due to ironoxides. After reduction of the 4-carboxybenzene diazoniumtetrafluoroborate 3, the spectrum of the mild steel plate has a C_((1s))peak at 289 eV at the same time as O_((1s)) signals at 531 and 532 eVattributed respectively to the carbon and to the oxygen of the C═O andof the OH of the carboxyl group as well as an F_([1s]) peak around 690eV due to surface contamination by BF₄ ⁻ ions of the electrolyte (FIG. 6c). When the mild steel plate is modified with 4-iodophenyl groups thesignal from the iodine atom can be observed at 621 eV (4.2%) (FIG. 6 d).The C/I ratio of 6.25 is close to what is to be expected for aniodophenyl group. In order to verify that the presence of the organicgroups on the surface is not due to simple adsorption or to a simplechemical reaction, the same mild steel plate, polished and cleaned aspreviously, was immersed in a solution of diazonium salt 2 in ACN+0.1MNBu₄BF₄. Under these conditions a weak is observed signal correspondingto iodine (0.45%). This weak signal may be due to a spontaneous reactionof diazonium salt 2 by the iron, as explained above. The attenuation ofthe intensity of the peak of Fe_([2p]) by the organic layer grafted byreduction of diazonium salt 1, 2 or 3 should be noted. A notableincrease in background noise is also observed, especially in the case ofgrafting of diazonium salt 2. This molecule seems to have a strongcovering power since we no longer observe the doublet of Fe_([2p]) inthe general spectrum. In its place, we observe in the 700–1000 eVregion, an increased background noise corresponding to an increased lossof inelastic energy.

XPS spectra were also employed for verifying that grafting did indeedoccur on the surface of the iron and not on the oxide, which may alwaysbe present on the surface. Two plates were passivated in 1N H₂SO₄ withsweeping of the potential up to +1V/SCE (a potential that is located inthe passivation zone of iron); passivation by the oxide layer can thenbe observed as an increase of the oxygen peak from about 20% to about40%. One of them was immersed in a solution of diazonium salt 2 inACN+0.1M NBu₄BF₄ whereas the other was grafted as previously in the samesolution. In both cases we only observe a weak signal from iodinecorresponding to 0.3 and 0.4% respectively. These signals are muchsmaller than those observed (4.2%) on a polished and previously cleanedplate that was then grafted as above. This clearly indicates that thegrafting of aryl groups does indeed take place on the iron and not onthe oxide layer.

Electrode capacitance: Attachment of an organic layer to the surface ofthe electrode should lower the capacitance of the said electrode byadding an additional insulating layer in series with the double layer.The capacitance can be measured by an impedance method; we recorded theNyquist diagram (Z_(Im) vs. Z_(Re)) (FIG. 7). It is then possible toobtain C_(d) from the frequency of the maximum^([15]). The capacitanceof a pure iron electrode of 3 mm diameter in 0.1N H₂SO₄ is 207 μF/cm².After modification of the electrode with 4-hexadecyloxyphenyl groups (byreduction of diazonium salt 8 in 0.1N H₂SO₄) the falls to 122 μF/cm².

Surface concentration of grafted groups: Two methods can be used formeasuring the surface concentration of aryl groups. In the first it isnecessary to measure (after complete rinsing of the electrode andtransferring it to a solution of ACN+0.1M NBu₄BF₄) the charge used forreducing the nitro group grafted to the surface (by single-electrontransfer). This amounts to integration of the voltammogram recordedusing an electrode modified with 4-nitrophenyl groups^([1]). One of thedifficulties of this method is that it requires estimation of a baseline and of the end potential of the integration. The second method isbased on integration of the RBS spectra; in this case we must be certainthat only the grafted zone of the electrode is included in the beam andthat the surface is stable under the ion beam. Both methods producevalues that are relative to the geometric area (i.e. 1 cm² for a 1 cm×1cm plate) of the electrode, polished beforehand with 1 μm diamond paste.

The values in Table 1 lead to consistent results for comparableconditions. For example, grafting of 4-nitrophenyl and 4-iodophenylgroups, that are of comparable size, leads to similar surfaceconcentrations, but if we increase the size of the group attached, goingfrom 4-iodophenyl to anthracenyl or to anthracenyl-9,10-dione, thesurface concentration decreases. When grafting is carried out in ACN,the surface concentrations are higher than the values obtained in 0.1NH₂SO₄, this may be due to oxidation of the surface in the sulphuricacid, which decreases the area available for grafting the aryl groups,or to hydrogen atoms formed on the surface or that have penetrated themetal^([16]) and which, by changing the surface properties, would makethe electrochemical reaction more difficult. The surface concentrationsshown in Table 1 can be compared with those measured previously oncarbon and in particular on HOPG (the geometric area of which is veryclose to the actual area).

TABLE 1 Surface concentration of aryl groups^([a]) Electrolysis Surfacepotential Method of concentration Sample Solvent^([c]) V/SCE measurementmol/cm^(2 [a]) Iron + 1 ACN −0.85 CV 45 10⁻¹⁰ Iron + 11 ACN −0.85 CV 1210⁻¹⁰ Iron + 12 ACN −0.85 CV  8 10⁻¹⁰ Steel + 2 ACN −0.85 RBS 48 10⁻¹⁰Steel + 2 0.1N H₂SO₄ −0.50 RBS 25 10⁻¹⁰ Steel + 2 ″ −0.65 RBS 22 10⁻¹⁰Steel + 2 ″ −0.75 RBS 32 10⁻¹⁰ Steel + 2 ″ −0.85 RBS 15 10⁻¹⁰ Steel — —RBS 0 ^([a]) Surface concentration (referred to the geometric area).^([b]) Electrode material + diazonium salt used for grafting. ^([c])Used for grafting.

The surface concentration of 4-nitrophenyl groups is from 12 to 18 10⁻¹⁰mol/cm^(2 [1,2]) on HOPG, but on vitreous carbon, polished under thesame conditions as the iron plates, a value of 40 10⁻¹⁰ mol/cm² wasobtained by reduction of diazonium salt 1. Therefore there is goodagreement between the values obtained on steel and on carbon. Thisresult was compared with the consumption of charge required for graftingby recording the chronopotentiometric curves (E=f(t) at constantcurrent). In the case of grafting of diazonium salt 5 the firsttransition time corresponds to reduction of the diazonium function.Practically identical transition times were measured on carbon and oniron. This indicates that the same quantity of charge is required toobtain complete covering of the electrode up to the time when thediazonium is no longer reduced on the electrode. On HOPG it wasdemonstrated previously that the surface concentrations obtainedcorrespond to a compact monolayer of aryl groups perpendicular to thesurface^([1]), a stereochemical situation in which the 4-nitrophenylgroup and the 4-iodophenyl group occupy the same surface and theanthracenyl groups occupy a larger surface.

Influence of the grafted layer on the corrosion of iron and mild steel.The effect of the organic layer was assessed by measuring twoparameters: the polarization resistance R_(p) and the corrosion currenti_(corr). Before making any measurements, the electrodes of iron or ofmild steel were carefully polished and pickled at constant cathodecurrent in HClO₄ ^([17]) then left to stabilize in the measurementmedium for about three hours at the open-circuit voltage, the value ofwhich can vary over time^([17]). The polarization resistances wereobtained either from the gradient of the curve obtained in anodic andcathodic sweeps of the potentials at 0.1 mV/s using the method of Sternand Geary^([18,19]) or from the impedance diagrams. The real andimaginary impedances were obtained from Nyquist diagrams (Z_(Im) as afunction of Z_(Re) for variable values of signal frequency). Thesediagrams allow the polarization resistance at low frequency and thecapacity at maximum Z_(Im)[20,21] (FIG. 7) to be obtained.

In order to lower the rates of corrosion, diazonium salts (5, 6, 7, 8)with long alkyl chains were selected so that the long chains provide ahydrophobic barrier that might limit the diffusion of oxygen and ofprotons. Similarly, compound 9 ought to constitute a very hydrophobicbarrier. The results are presented in Table 2.

TABLE 2 Polarization resistance and corrosion currents on modifiedelectrodes of iron or mild steel Diazo- nium Corrosive R_(p) ^([b])R_(p) ^([c]) i_(corr) E _(corr) Sample salt^([a]) medium Ω Ω μA/cm² mV^([d]) R_(I) ^([e]) Iron None 0.1N H₂SO₄ 1924 1600 179 −475 — Iron 50.1N H₂SO₄ 2206 2307 117 −520 35 Iron 6 0.1N H₂SO₄ 2958 2721 101 −523 44Iron 8 0.1N H₂SO₄ 2042 2610 84 −541 53 Iron 9 0.1N H₂SO₄ 2688 2500 125−527 30 Steel None 0.1N H₂SO₄ 1358 1513 168 −546 — Steel 5 0.1N H₂SO₄3198 4819 48 −515 62 Steel 8 0.1N H₂SO₄ 3126 3627 43 −530 66 Iron None5% NaClO₄ 4317 4604 37 −690 — Iron 5 5% NaClO₄ 8205 9254 13 −637 64 Iron8 5% NaClO₄ 7167 7817 13 −636 64 Iron None 5% KCl 3353 4319 34 −692 —Iron 5 5% KCl 8999 9403 6 −738 82 ^([a])used for grafting.^([b])obtained from the curves i = f(E). ^([c])obtained from theimpedance curves. ^([d])in mV/SCE. ^([e])efficiency of inhibition.

In the three media used, an increase in polarization resistance and adecrease in corrosion current through grafting of the surfaces of ironor of mild steel is observed. There is a good correlation between theresults obtained for the polarization resistance measured by the twodifferent methods. Table 2 includes the efficiency of inhibition RIcalculated from: RI=100 (1−i_(cor) ^(graft)/i_(cor)). The corrosion rateis decreased both in the acidic medium, where the protons are the mainoxidizing species, and in the neutral medium, where oxygen isresponsible for corrosion. In an acidic medium, the corrosion potentialis shifted towards the cathode potentials, which can be attributed topredominant cathodic inhibition^([23]). In a neutral medium thecorrosion potential is shifted anodically, indicating that the organicfilm makes oxidation of the iron more difficult.

The stability of the grafted layer was measured as a function of theelectrode potential. The potential of an iron electrode modified with4-iodophenyl groups (by reduction of diazonium salt 2 in a solution ofACN+0.1M NBu₄BF₄) was swept anodically from 75 and 150 mV at 1 mV/sstarting from the corrosion potential in 0.1N H₂SO₄. Then the surfacewas examined by XPS and RBS: no significant change in surfaceconcentration of iodine is observed for a sweep of 75 mV, but the latterdrops to 0 for a sweep of 150 mV. On a mild steel electrode of 0.07 cm²,an anode sweep of 75 mV corresponds to the consumption of 9.8 mC or 510⁻⁸ mol of iron dissolved (which was verified by assaying the solutionby atomic absorption spectroscopy). This shows that during an anodesweep, the organic groups are detached from the surface at the same timeas iron atoms.

The results of the invention can also be compared with the adsorption,on iron, of long-chain (C₁₂, C₁₈) alkanethiols subjected to the actionof octyltriethoxysilane so as to increase the length of the chains andat the same time provide side bonding between the chains.^([17]) In thecase of a C18 alkanethiol in a weakly aggressive medium such as 0.5MNaCl, the transfer resistance increases about 4 times, and about 4.5times after reaction with octyltriethoxysilane. However, a majordrawback of this method is that the thiol is desorbed slowly from thesurface. Therefore this surface modification is not stable. The organiclayer is not firmly grafted and can be removed in an ultrasonic tank,whereas the layer grafted in accordance with the method of the inventionwithstands this treatment.

Metallic surfaces (Fe, Ni, Pt) were also modified covalently byelectrochemical reduction of vinyl monomers such as acrylonitrile,methacrylonitrile or butene nitrile. Thin films of polymers (≈10 to 50nm) bound covalently to the surface have been studied indetail^([24,25]) and have been found to be effective for corrosionprotection^([26]). The most notable difference between this method andthe method of the invention is that the grafted layer is of the alkyltype, the first carbon bound to the metal is a saturated aliphaticcarbon whereas according to the invention it is an aromatic carbon thatis bound to the metal. From this standpoint, the two methods are quitedifferent; the method of the invention does not make it possible tograft an aliphatic carbon since the aliphatic diazonium salts areunstable and the method using vinylic monomers does not permit thegrafting of an aromatic carbon. Other differences are as follows. a) theinvention makes it possible to graft unpolymerized monolayers whereasthe organic layer obtained from vinyl monomers is necessarily a polymer;b) the method employing vinyl monomers involves the use of apolymerizable monomer, whereas the invention permits preforming of thepolymer (which permits any changes of structure), place it in solutionand then attach it by chemical reaction on an organic layer previouslygrafted to the metal surface; c) the invention permits preforming of apolymer possessing aminophenyl groups, transformation of these aminogroups to diazonium salts then grafting the polymer thus functionalized,which is impossible in the case when vinyl monomers are used.

The organic films obtained by reduction of diazonium salts thereforereduce corrosion, as shown in Table 2; they are stable and withstand ananodic excursion of 75 mV.

Example II

Grafting of Zinc

The grafting of compounds 1, 5 and 10 was investigated on this metal,which is used in the production of galvanized sheets of mild steel forcars. The first and the last can easily be characterized, whereasdiazonium salt 5 can provide hydrophobic protection of the metal againstcorrosion.

FIGS. 8 a, 8 b and 8 c show the voltammogram of anthracene (in ACN+0.1MNBu₄BF₄) on a carbon electrode (8 a), on a zinc electrode (8 b) andfinally of an electrode grafted at −1.1 V/SCE in a 2 mM solution of 10in ACN+0.1M NBu₄BF₄, carefully rinsed in an ultrasonic tank andtransferred to a solution containing only acetonitrile and the base salt(8 c). The reduction of anthracene on zinc is reversible, as on carbon(Epc=−2.09 V/SCE, Epa=−1.93 V/SCE, ° E=−2.01 V/SCE); the wave observedafter grafting is located at −2.2 V/SCE, i.e. at a potential close tothat of anthracene, on the other hand the wave has lost itsreversibility almost entirely, for a reason that has yet to beelucidated. The presence of this wave on the grafted and carefullyrinsed electrode clearly indicates the presence of a reducible groupattached to the surface, and the similarity of the peak potentials meansthat this signal can be ascribed to an anthracenyl group grafted on thesurface. The same phenomenon is observed if grafting takes place indilute sulphuric acid.

The same phenomenon can be observed on grafting 4-nitrobenzenediazonium 1. FIGS. 9 a and 9 b show the reversible wave on the carbon ofthe nitrobenzene (° E=−1.23 V/SCE) and FIG. 9 b shows a zinc electrodegrafted with 4-nitrophenyl groups; this wave is wide and reversible, andits standard potential is identical to that of the nitrobenzene insolution on a carbon electrode. This clearly shows that grafting of4-nitrophenyl groups has indeed taken place.

When a zinc electrode grafted with 4-butylphenyl groups (by reduction of5) is placed in 0.1N H₂SO₄ solution and the potential is sweptanodically (v=20 mV/s), the anode current observed is about 30% lowerthan on an untreated electrode, which points to a certain degree ofprotection of the zinc surface by the organic layer.

Example III

Grafting of Copper

Copper is oxidized electrochemically towards 0 V/SCE in ACN and towards−0.15 V/SCE in 0.1N H₂SO₄, therefore reduction of diazonium saltstowards −0.5 V/SCE can be carried out in both media. FIGS. 10 a, 10 band 11 a, 11 b show a copper electrode in a solution of anthraquinoneand of nitrobenzene respectively, then the same electrode grafted withanthraquinone and nitrophenyl groups. The similarity of the potentialsof the reversible systems is evidence of grafting of the copper.

Example IV

Grafting of Nickel

Nickel is oxidized towards +0.1 V/SCE in ACN and in 0.1N H₂SO₄. It istherefore possible to graft nickel electrodes towards −0.5 V/SCE. FIGS.12 a and 12 b, similar to FIGS. 11 a and 11 b for copper, provideevidence of grafting on a nickel surface (on the grafted electrode aprewave is observed at a less negative potential than the wave of the4-nitrophenyl group). The similarity of the potentials of the reversiblesystems indicates that grafting does indeed occur on nickel.

Example V

Grafting of Cobalt

Cobalt is oxidized towards −0.2 V/SCE in ACN and towards −0.3 in 0.1NH₂SO₄. It is therefore possible to graft cobalt electrodes towards −0.5V/SCE. FIGS. 13 a and 13 b, similar to FIGS. 11 a, 11 b (on copper) and12 a, 12 b (on nickel), provide evidence of grafting on a cobaltsurface.

Example VI

Grafting of Gold

On gold, which is difficult to oxidize, we can observe not only the waveof nitrobenzene (FIG. 14 b) as in the preceding cases, but also the waveof reduction of diazonium towards +0.2 V/SCE (FIG. 14 a) and finally thewave of the grafted 4-nitrophenyl group (FIG. 14 c). Therefore graftingis carried out towards 0 V/SCE and the similarity of the potentials ofthe reversible systems of FIG. 14 b and c clearly indicates grafting ofthe 4-nitrophenyl group on gold.

Example VII

Grafting of Platinum

The same curves can be observed on platinum. Grafting carried outtowards 0 V/SCE can be confirmed by XPS as shown in Table 3: thenitrogen of the NO₂ group can be clearly seen at 406 eV which increasesin glancing incidence, which clearly shows that it is on the outside ofthe layer and the almost total disappearance of the signal fromplatinum, masked by the organic layer.

TABLE 3 XPS spectra of a platinum surface grafted with 4-nitrophenylgroups. Elements C_(1s) O_(1s) N_(1s) total N_(1s)(NO₂₎ Pt_(4f) Pt, bare26.8 16.3 0 0 56.8 Pt, grafted 1) 64.7 20.7 14.35 7.2 0.33 2) 64.3 18.916.8 9.5 0.00 1) normal incidence, 2) glancing incidence

Example VIII

Grafting of Titanium

FIGS. 16 a, 16 b show the voltammogram of diazonium salt 1 (Epc=−0.2V/SCE) on a polished titanium electrode (d=1 mm) in a solution ofACN+0.1M NBu₄BF₄; this irreversible wave disappears during the secondsweep (FIG. 16 a) just as was observed during grafting on carbonsurfaces. Therefore it is possible to effect grafting towards −0.5V/SCE. After careful rinsing in acetone in an ultrasonic tank, thiselectrode is then transferred to a solution containing only acetonitrileand the supporting electrolyte. We then observe the reversiblevoltammogram of the nitrophenyl group grafted on the surface at °E=−1.11 V/SCE (FIG. 16 b), very close to that of nitrobenzene itself ona carbon electrode. The wave of the grafted 4-nitrophenyl group ispreceded by a prewave, similar to that observed on carbon, the origin ofwhich has not yet been elucidated.

The XPS spectrum of the surface confirms grafting of the surface with4-nitrophenyl groups.

TABLE 4 XPS spectra of a titanium surface grafted with 4-nitrophenylgroups. Ti % 459 and 464 eV C % O % N % Polished 33.5 23.0 42.6 401 eV1.0 titanium plate 533 eV 2.7 406 eV 0 Grafted 14.9 42.0 38.4 401 eV 1.6titanium plate 533 eV 15.3 406 eV 3.3

After grafting of the plate, a decrease of the signal from titanium isobserved due to the presence of the organic layer, an increase of thecarbon (on the polished plate, carbon comes from contaminants), anincrease of the oxygen of the nitro group at 533 eV and appearance ofthe signal from nitrogen at 406 eV (3.3%), characteristic of the nitrogroup.

Example IX

Grafting of Stainless Steel

Stainless steel 316 is commonly used in prostheses. FIGS. 17 a, 17 b and17 c show, in the same way, the voltammogram of 1 on an electrode ofstainless steel 316 (FIG. 17 a), of the 4-nitrophenyl groups grafted byelectrolysis in ACN+0.1M NBu₄BF₄+2 mM 1 at −0.2 V/SCE (FIG. 17 b) and ofnitrobenzene in the same solution (FIG. 17 c). The XPS spectrum confirmsthe grafting.

TABLE 5 XPS spectra of a surface of stainless steel 316 L grafted with4-nitrophenyl groups. Fe % 707 and 710 eV C % O % N % Polished steel52.4 17.3 29.1 401 eV 1.22 plate 533 eV 3.6 406 eV 0 Grafted steel 10.456.3 26.5 401 eV 1.8 plate 533 eV 12.3 406 eV 5.0

After grafting of the plate, weakening of the signal from iron isobserved, due to the organic layer, an increase of the signal due to thecarbon, a strong increase of the signal from oxygen at 533 eV and of thesignal from nitrogen at 406 eV, both of which are characteristic of thenitro group.

Cyclic voltammetry and XPS spectroscopy therefore both definitelyconfirm the existence of grafting on the stainless steel.

Comparative Examples Comparative Example No. 1

Comparison of FIGS. 19 and 20 demonstrates the importance of thepresence of a diazonium group.

FIG. 19 was obtained by sweeping the iron electrode between −0.6 V and−1.8 V in the presence of nitrobenzene (concentration 1 mM) in asolution of acetonitrile+base salt (0.1M N₄BuF₄). Then twenty sweeps arecarried out between the same potentials. The method employed is the sameas that used for grafting the diazonium salts.

Then, after rinsing the electrode for 5 minutes in acetone and 5 minutesin chloroform in an ultrasonic tank, a new test is carried out,consisting of bringing this electrode into contact with a solutioncontaining only acetonitrile and the base salt, and not containingnitrobenzene, and recording the corresponding voltammogram (FIG. 20). Nosignal is observed: the nitrobenzene function has not been grafted onthe electrode even though the method employed was the same as was usedfor grafting diazonium salts. This therefore shows that the diazoniumgroup is essential for grafting.

Comparative Example No. 2

Experiments similar to those in Comparative Example No. 1 were carriedout with p-nitrophenol (FIGS. 21 and 22).

FIG. 21B was obtained by sweeping the iron electrode between −0.6 V and−1.6 V in the presence of p-nitrophenol (concentration 1 mM) in asolution of acetonitrile+base salt (0.1M N₄BuF₄). Then twenty sweeps arecarried out between the same potentials. The method used is the same aswas used for grafting the diazonium salts. (FIG. 21A corresponds to theblank electrode).

Then, as previously, after rinsing the electrode for 5 minutes inacetone and 5 minutes in chloroform in an ultrasonic tank, a newexperiment is carried out, consisting of placing this electrode incontact with a solution containing only acetonitrile and the base saltand not containing p-nitrophenol, and recording the correspondingvoltammogram (FIG. 22). No signal is observed: the p-nitrophenol grouphas not been grafted on the electrode even though the same method wasfollowed as was used for grafting the diazonium salts. This thereforeshows that the diazonium group is essential for grafting.

Conclusion

It has been shown that the reduction of diazonium salts on metallicelectrodes: iron, mild or stainless steel, zinc, copper, nickel, cobalt,titanium, gold or platinum, leads to grafting of the aryl group on thesurface. The presence of these groups has been demonstrated by cyclicvoltammetry, XPS, PMIRRAS, RBS and by capacitance measurements. Thesemethods are based on different physical principles, so the convergenceof the results demonstrates the reality of grafting. In all cases, thisgrafting withstands vigorous rinsing in an ultrasonic tank, indicatingthe existence of strong bonds, beyond simple adsorption. Themeasurements by cyclic voltammetry and RBS show that the molecular filmis close to being a monolayer. These organic layers, especially thosehaving an alkyl chain as substituent, present a cetain effectivenessagainst corrosion.

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1. A metallic material, the surface of which is modified by bondingaromatic groups to said surface, optionally substituted by functionalgroups, the nature of the bond between the surface and the aromaticgroups that modify said material being a carbon-metal bond of covalenttype, and wherein said material withstands ultrasonic cleaning and saidmaterial is made by electrochemical reduction of a diazonium saltcontaining said aromatic group in the presence of said metallicmaterial.
 2. The metallic material according to claim 1, in which thenature of the bond between its surface and the aromatic groups thatmodify the latter is such that when an anode sweep is carried outstarting from the corrosion potential of the metal forming the surfaceof the metallic material, there is at least one potential responsiblefor the dissolution of a detectable quantity of the aforesaid metal butwhich does not disrupt the aforesaid bond.
 3. The metallic materialaccording to claim 2, in which the nature of the bond between itssurface and the aromatic groups that modify the latter is such that whenan anode sweep is applied ranging from the corrosion potential to apotential that is more anodic by about 75 mV, to the metallic materialwhose surface is modified, rupture of the aforesaid bond does not occur,but there is dissolution of a detectable quantity of the metal formingthe surface of the metallic material.
 4. The metallic material accordingto claim 1, in which the aromatic group is a C₆–C₁₄ aromatic residue,optionally substituted by one or more functional substituents or aheteroaromatic residue of 4 to 14 atoms, optionally substituted by oneor more functional substituents, comprising one or more heteroatomsselected from oxygen, nitrogen, sulphur or phosphorus.
 5. The metallicmaterial according to claim 1, in which the aromatic group has one ormore substituents capable of reacting directly with a substrate, or oneor more precursor substituents which, after transformation, are capableof reacting with a substrate, the said substrate being selected from thegroup comprising organic resins, biological molecules, chemicalmolecules or complexing agents.
 6. The metallic material according toclaim 1, in which the aromatic group has one or more substituents thatare capable of reacting directly with a biological molecule and areselected from the group comprising (CH₂)_(n)—COOH, (CH₂)_(n)—NH₂, nbeing an integer comprised between 0 and 10, or one or more precursorsubstituents that are capable of reacting after transformation with abiological molecule and are selected from the group comprising NO₂, N₂⁺, (CH₂)_(n)—CN, (CH₂)_(n)—CHO, (CH₂)_(n)—COOPr, Pr being a protectivegroup, n being an integer comprised between 0 and
 10. 7. The metallicmaterial according to claim 1, in which the aromatic group has one ormore substituents that are capable of reacting directly with functionalorganic molecules and are selected from the group comprising NO₂,(CH₂)_(n)—CONH₂, (CH₂)_(n)—CN, (CH₂)_(n)—CHO, (CH₂)_(n)—COOH,(CH₂)_(n)—CH₂OH, (CH₂)_(n)—NH₂, n being an integer comprised between 0and 10, SO₂H, SO₃H, SO₂R, SO₃R, R being an aliphatic or aromatic carbonchain with 1 to 20 carbon atoms, or one or more precursor substituentsthat are capable of reacting after transformation with functionalorganic molecules and are selected from the group comprising NO₂,(CH₂)_(n)—CONH₂, (CH₂)_(n)—COOPr, Pr being a protective group,(CH₂)_(n)—NHP′r, (CH₂)_(n)—N (P′r)₂, (CH₂)_(n)—N═P″r, P′r and P″r beingprotective groups, (CH₂)_(n)—CN, (CH₂)_(n)—CHO, (CH₂)_(n)—COOH,(CH₂)_(n)—CH₂OH, n being an integer comprised between 0 and10, SO₂Pr,SO₃Pr, Pr being a protective group selected from the meanings of R. 8.The metallic material according to claim 1, in which the aromatic groupis selected from:


9. The metallic material according to claim 1, in which the metal isselected from pure metals or alloys, and in particular iron, nickel,platinum, gold, copper, zinc, cobalt, titanium, chromium, silver,stainless steels, titanium alloys, chromium-cobalt alloys, molybdenum,manganese, vanadium.
 10. The metallic material according to claim 1, thesurface of which is modified by an organic layer whose minimum thicknessis about 10 Å.
 11. A metallic material, the surface of which is modifiedby bonding aromatic groups to said surface, optionally substituted byfunctional groups, the nature of the bond between the surface and thearomatic groups that modify said material being a carbon-metal bond ofcovalent type, and wherein said material withstands ultrasonic cleaningand said material is made by electrochemical reduction of a diazoniumsalt containing said aromatic group in the presence of said metallicmaterial and wherein the aromatic group has one or more substituentsselected from the group comprising: linear or branched aliphaticradicals of 1 to 20 carbon atoms, optionally containing one or moredouble or triple bonds, optionally substituted by the followingradicals: carboxyl, NO₂, disubstituted protected amino, monosubstitutedprotected amino, cyano, diazonium, alkoxy of 1 to 20 carbon atoms,alkoxycarbonyl of 1 to 20 carbon atoms, alkylcarbonyloxy of 1 to 20carbon atoms, vinyl optionally fluorinated or allyl, halogen atoms, arylradicals optionally substituted by the following radicals: carboxyl,NO₂, disubstituted protected amino, monosubstituted protected amino,cyano, diazonium, alkoxy of 1 to 20 carbon atoms, alkoxycarbonyl of 1 to20 carbon atoms, alkylcarbonyloxy of 1 to 20 carbon atoms, vinyloptionally fluorinated or allyl, halogen atoms, the following radicals:carboxyl, NO₂, disubstituted protected amino, monosubstituted protectedamino, cyano, diazonium, alkoxy of 1 to 20 carbon atoms, alkoxycarbonylof 1 to 20 carbon atoms, alkylcarbonyloxy of 1 to 20 carbon atoms, vinyloptionally fluorinated, halogen atoms.
 12. A metallic material, thesurface of which is modified by bonding aromatic groups to said surface,optionally substituted by functional groups, the nature of the bondbetween the surface and the aromatic groups that modify said materialbeing a carbon-metal bond of covalent type, and wherein said materialwithstands ultrasonic cleaning and said material is made byelectrochemical reduction of a diazonium salt containing said aromaticgroup in the presence of said metallic material, and wherein thearomatic group has one or more substituents that are capable of reactingdirectly with an organic resin and are selected from the groupcomprising allyl or vinyl groups, —(CH₂)_(n)—COOR, —(CH₂)_(n)—CH₂—OH,(CH₂)_(n)—NH₂, n being an integer comprised between 0 and 10, or one ormore precursor substituents that are capable of reacting aftertransformation with an organic resin and are selected from the groupcomprising NO₂, N₂ ⁺, (CH₂)_(n)—CN, (CH₂)_(n)—CHO, (CH₂)_(n)—COOPr, Prbeing a protective group, (CH₂)_(n)—NHP′r, (CH₂)_(n)—N(P′r)₂,(CH₂)_(n)—N═P″r, P′r and P″r being protective groups, n being an integercomprised between 1 and
 10. 13. A metallic material obtained by a methodof modifying the surface of a metallic material, in order to obtain ametallic material according to claim 1, comprising attaching an aromaticgroup to the surface of the said material, by electrochemical reductionof a diazonium salt containing this aromatic group, by bringing themetallic material into contact with a solution of the diazonium salt ina solvent and by negatively polarizing the metallic material relative toan anode that is also in contact with the solution of the diazoniumsalt, and wherein the anode and the cathode may be separated from oneanother.
 14. The metallic material according to claim 13, characterizedin that electrochemical reduction takes place in the presence of anelectrolyte, the anode and cathode compartments optionally beingseparated, the anode compartment containing the solvent and theelectrolyte, and the cathode compartment containing the solvent, theelectrolyte and the diazonium salt.
 15. The metallic material accordingto claim 13, characterized in that the diazonium salt corresponds to theformula ArN₂ ⁺ X⁻, in which Ar represents the aromatic group and Xrepresents an anion and in that this diazonium salt has a reductionpotential that is less negative than the reduction potential of the freeradical Ar° corresponding to the aromatic group of the diazonium salt,the anion X⁻ of the diazonium salt being selected advantageously fromthe halogens, the sulphates, the phosphates, the perchlorates, thetetrafluoroborates, the carboxylates, and the hexafluorophosphates. 16.The metallic material according to claim 13, characterized in thatreduction is carried out by repetitive cyclic voltammetry in a potentialrange where the diazonium salts are reduced either by electrolysis at apotential that is more negative than the reduction potential of thediazonium salt, or at constant current.
 17. The metallic materialaccording to claim 13, characterized in that the concentration ofdiazonium salts is between 10⁻³ and 10⁻¹ mol/l.
 18. The metallicmaterial according to claim 13, for the production by electrochemicalmeans, of a metallic material whose surface is modified with aromaticamino groups, characterized in that the aromatic diazonium salt issubstituted by a nitro radical and in that electrochemical reduction ismaintained until there is reduction of the nitro radical to aminoradical.
 19. The metallic material according to claim 13, in whichelectrochemical reduction of the diazonium salt takes place in anaprotic solvent, selected from the group consisting of acetonitrile,dimethylformamide, dimethylsulphoxide and benzonitrile.
 20. The metallicmaterial according to claim 19, characterized in that the solution ofthe diazonium salt contains a supporting electrolyte consisting of aquaternary ammonium salt or a lithium salt, especially atetraalkylammonium tetrafluoroborate.
 21. The metallic materialaccording to claim 13, in which electrochemical reduction of the organicdiazonium salt takes place in a protic solvent in an acid medium. 22.The metallic material according to claim 21, characterized in that theprotic solvent is selected from the group consisting of water, methanol,ethanol and their mixtures, or in that the protic solvent is mixed withan aprotic solvent, provided the resulting mixture has thecharacteristics of a protic solvent.
 23. The metallic material accordingto claim 21, characterized in that the acid is selected from the groupconsisting of sulphuric, hydrochloric, nitric, nitrous, phosphoric andtetrafluoroboric acids.
 24. The metallic material according to claim 21,characterized in that the pH of the solution is less than 2.